Through-silicon vias (TSV), also commonly referred to as through-wafer vias (TWV), are widely used in integrated circuits. TSVs can be used for stacking dies. In stacked dies including a first die stacked on a second die, electrical connections may be made from the first die to the top surface of the second die through TSVs in the first die. TSVs are also commonly used for providing a quick and low-resistive path from a front surface of a die to its back surface.
FIGS. 1 through 3 illustrate cross-sectional views of intermediate stages in a conventional process for forming TSVs in a die and grounding the die through the TSVs. Referring to FIG. 1, TSVs 12 are formed in die 10, and extend from substantially the front surface (the top surface) of die 10 into substrate 14, which are typically formed of silicon. TSVs 12 may be used to provide a grounding path for integrated circuit in die 10. Due to process variations, TSVs 12 may have different lengths L.
In FIG. 2, the back surface of die 10 is polished using a slurry that attacks silicon. To ensure all TSVs 12 are exposed through the back surface, silicon substrate 14 may have to be polished to a level so that at least some of the TSVs 12 slightly protrude out of the back surface. Due to the different lengths L of TSVs 12, some of the TSVs 12 will protrude more than others.
In FIG. 3, die 10 is mounted on substrate 16, which may be a lead-frame or other types of package substrates, through silver paste 18. Typically, silver paste 18 is in liquid form, and is applied on substrate 16, followed by pressing die 10 against silver paste 18. Silver paste 18 is conductive, and hence electrically interconnecting TSVs 12. Die 10 is thus grounded through TSVs 12. Since TSVs 12 may protrude out of the back surface of die 12 for different lengths L, die 10 may be tilted. This causes reliability issues. For example, after the formation of wire bonds, molding compound 20 will be applied. Due to the tilting of die 10, some portion of the wire bonds, for example, wire 22, may not be fully covered by molding compound 20, and hence is subject to mechanical damage.
One way to solve the above-discussed problem is to polish back the protruded TSVs 12 after the polishing of silicon substrate 14. However, since TSVs 12, which are typically formed of copper, need different slurries than silicon substrate 14, an additional polishing process is needed. Further, the process for polishing copper is more costly than polishing silicon due to contamination issues. Therefore, polishing back TSVs is a undesirable solution. Accordingly, new methods for packaging dies on substrates without causing the above-discussed problem are needed.